The present invention relates to photons detectors, radiation detectors, scintillation counters, gamma cameras and more particularly to photomultiplier tube bases that are used in such cameras and detectors.
A photomultiplier tube (PMT) is a photosensitive device that converts light photons into a electrical current. The main components of a PMT are an input window, a photocathode, focusing electrodes, dynodes and an anode (output). The photocathode is used for converting incoming light (photons) into electrons. These photoelectrons, which are a product of photoelectric effect, are directed by the potential of focusing electrodes towards dynodes. The dynodes are used to multiply the electrons by the process of secondary electron emission. Electron gains of 103 to 108 are common and depend on the number of dynodes and inter-dynode potentials. Dynodes are made or covered with a layer of secondary emissive material. The condition of the dynode surfaces are responsible for PMT stable gain performance. All known dynode emissive materials are sensitive to electron stress. The most sensitive dynodes are those that are at the end of the series of dynodes, where the quantity of secondary electrons emitted is the largest. Understandably, for long-term, stable operation of a PMT, a low anode currents is preferable.
The voltages that create the electrostatic fields between the photocathode, the focusing electrodes and the dynodes are delivered from a single high-voltage stable power supply and a voltage divider. The divider is a common part of a PMT base. The design of the divider circuit is crucial to getting the best performance from PMT. There are many versions of PMT high voltage dividers optimized or designed for some particular application. Most of them are concentrated on specific parameters that are critical for a given application, such as maximum gain, dynamic range, low noise, or linearity.
Series-regulator type high voltage power supplies optimized for photomultiplier tubes are well known in the art and have gained a good reputation. Other components found in or required by scintillation cameras are described in xe2x80x9cPhotomultiplier Tube, Principle to Applicationxe2x80x9d by Hamamatsu Photonics K. K., March 1994, which is incorporated herein by reference.
The output of a photomultiplier tube is a current (charge), while the external signal processing circuits are usually designed to handle a voltage signal. Therefore, the current output must be converted into a voltage signal by a current to voltage converter. Further, the current that is output from a PMT anode is very small, especially in low light level detection, low gain PMT""s, and photon counting applications. An operational amplifier can be used to both convert the anode output current to a voltage and accurately amplify the resulting voltage. Typically this operational amplifier is powered by a source that is separate from the high voltage power source for the dynode stages of the PMT. This is done to insure the stability of the power supply to the dynodes. The voltage supplied to each dynode stage must be extremely stable or the PMT output will be adversely affected. Prior PMT bases contained only components for the dynode stages of the PMT. It was thought that by keeping the PMT base simple, it would be easier to maintain stable power.
U.S. Pat. No.5,864,141 entitled, xe2x80x9cCompact, high-resolution, gamma ray imaging for scintimammography and other medical diagnostic applicationsxe2x80x9d discloses a typical PMT system wherein the amplifier of the PMT output signal is powered by a power source that is external to the PMT system and PMT high voltage divider.
U.S. Pat. No. 5,525,794 entitled, xe2x80x9cElectronic gain control for photomultiplier used in gamma cameraxe2x80x9d discloses active components that are powered by the PMT power source, however, these active components only provide gain control for the dynode stages and do not amplify the PMT output signal.
U.S. Pat. No. 5,367,222 entitled, xe2x80x9cRemote gain control circuit for photomultiplier tubesxe2x80x9d is similar to the ""794 Patent above. Both Patents disclose the PMT high voltage divider powering only active components for gain control. All active components, in both Patents, for amplifying the PMT output signal are powered by an external power source.
The present invention provides a PMT base with an active component that is not used for high voltage stability but rather for PMT output pulse amplification, pulse shaping and device impedance conversion. In the preferred embodiment an amplifier is integrated into and powered by the PMT high voltage divider circuit. This allows an improved signal to noise ratio and reduces PMT anode current there by providing better long-term stability when compared to prior PMT bases. The amplifier also provides improved pulse shaping for better signal transmission through long external cable lines. The present PMT base also minimizes the power cables that are required by the system. Further, and perhaps most importantly, the power supply to the dynode stages of the PMT remains stable, in the present PMT base.
Typically, a PMT provides reliable, stable gain for some hundreds or thousands of hours, then its stability decreases as a function of the total charge handled. The decreased stability is primarily a result of dynode degradation. The dynode stages of a PMT are worn down more quickly when higher voltages, higher gain and higher anode currents are used. The use of very high voltages across the dynode stages provides a PMT output that may not need amplification. However, this leads to frequent replacement of the PMT""s. Systems, like those mentioned in the patents above, use relatively low voltages between the dynode stages but then send the PMT output to a circuit that is external to the PMT for amplification. These systems have poor signal to noise ratios and have higher overall power consumption, when compared to the present system. By integrating the PMT output amplifier with the circuits that control the dynode stages, all of which are powered by the PMT high voltage divider, a PMT base is created that extends the life of the PMT and allows easy replacement of current PMT bases of scintillation cameras, gamma cameras and other radiation detectors without the need to worry about a second power supply for amplifying the PMT output.
The present innovative concept provides a PMT base with long-term stability, improved signal to noise ratios and proper long line termination without the need of an external amplifier for a PMT output signal. An amplifier for the PMT output is integrated with the circuits for the dynode stages in a way that keeps the power supply stable for the voltage sensitive dynodes. The PMT high voltage divider provides power sequentially to each dynode stage circuit and then to the PMT output amplification circuit. The PMT output signal from an anode, the last dynode, or both, provides the input for the present amplification circuit. The dynode circuits and the output amplification circuit are integrated into one unit that receives power from the PMT high voltage divider and produces an amplified PMT output signal without disrupting the stability of the power provided to the dynode stages. The amplification circuit may comprise any suitable component or components for amplifying the PMT output, such as a one stage or multistage transistor and/or resistor circuit, for example.